1. Field of the Invention
The present invention relates to an engine operated generator and particularly, to an engine operated generator capable of controlling the revolution of an engine depending on the magnitude of load.
2. Description of the Related Art
Engine operated generators for use as an AC (alternative-current) power sources have been known which employ inverters to stabilize the output frequency. Such an engine operated generator is joined to and driven by an engine to generate an AC power which is converted into a DC power and then returned back by the inverter to desired AC power of a commercial frequency. The engine operated generator equipped with the inverter permits its output frequency to hardly depend on the revolution of the engine and can thus control the engine revolution with the magnitude of load to determine its output.
For example, an inverter-equipped engine operated generator is disclosed in Japanese Patent Laid-open Publication Hei 5-18285 which detects a load from the output current of the inverter and uses its detected value to perform throttle control on the engine. That control technique allows the output voltage to be maintained substantially constant regardless of variations of the load.
For controlling the engine revolution to an optimum rate depending on the magnitude of load, it is essential that the load stays within the generation capability of the generator. However, the optimum rate of the revolution corresponding to the magnitude of the load has to be calculated using various parameters, for example, effective output power of the inverter (that is, voltage.times.current.times.power factor), inversion efficiency of the inverter, power generation capability per revolution, and tolerances of the generator and the effective power detector, hence making the control very difficult and complicated.
Another engine operated generator is proposed (as disclosed in Japanese Patent Laid-open Publication Hei 5-146200) where the output voltage of the generator is detected at an input of an inverter and compared with a predetermined reference voltage to determine a corresponding rate of the engine revolution to the load.
The disadvantage of the above two conventional engine operated generators is that once the output voltage of the generator has been declined by overload which exceeds an output corresponding to the current revolution of the engine, the recovery of the engine revolution to a target rate is significantly delayed.
Also, there is a time lag from the throttle opening action, which is performed on the basis of a difference between the detected output current or voltage and a target level, to the release of its corresponding output from the generator. This causes the engine operated generator to hardly respond to a quick change in the load and leave the output voltage without stability.